Tamoxifen was approved five years ago as the first drug to prevent breast cancer. It can halve the incidence of breast cancer in women who are most likely to develop the disease. As with all medicines, tamoxifen has side effects that may affect some women and not others. In this case, the effects are rare, but serious--endometrial cancer, stroke, deep vein thrombosis, and pulmonary embolism. The current study weighed these risks, which are especially high for older women, against the benefits of tamoxifen to determine how many women in the United States are likely to have a net benefit from the drug.
"Our study makes progress in evaluating the potential public health impact of tamoxifen use and identifies subgroups of women who may especially benefit from tamoxifen," said Freedman. "Although these findings suggest a benefit of tamoxifen for certain women, the choice to take tamoxifen is an individual one. Women with increased risk of breast cancer must carefully consider the benefits and risks in consultation with their physicians."
The decision to take tamoxifen will depend on a woman's age, breast cancer risk factors, family history, how she weighs the benefits and risks, and her specific medical situation, lifestyle, personal values, and preferences, said Worta McCaskill-Stevens, M.D., one of the co-investigators on the NCI study. "Tamoxifen therapy may not be appropriate for all women who are at increased risk for breast cancer," said McCaskill-Stevens.
Tamoxifen was approved as a chemoprevention drug for breast cancer by the Food and Drug Administration (FDA) in 1998, after NCI released the results of the Breast Cancer Prevention Trial (BCPT), a six-year study of the drug. In BCPT, tamoxifen was found to reduce the incidence of breast cancer by 49 percent. Based on that study, FDA approved the drug for women at high risk of developing invasive breast cancer. High risk was defined as women age 35 and older who have a five-year risk of at least 1.67 percent.
Using data on cancer risk factors from the 2000 National Health Interview Survey, Freedman and colleagues calculated the number of women in this country eligible to take tamoxifen based on FDA-approved indications. They also projected the number of white and black women who would most likely have a net positive benefit from taking the drug based on a benefit-risk analysis. Because accurate data on the frequency of adverse tamoxifen effects in Hispanic women were not available, estimates of how many Hispanic women would likely benefit from the drug could not be calculated, Freedman said.
The researchers estimated that 15.5 percent of women 35 to 79 years old in this country, or about 10 million, would be eligible to take tamoxifen based on breast cancer risk alone.
When analyzed by race, 18.7 percent of white women ages 35 to 79 in the United States, or 9.4 million, would be eligible for tamoxifen, but only 4.9 percent, or 2.4 million, are likely to benefit from the drug. About 6 percent of U.S. black women in the same age range, or 430,000, would have a high enough risk to take the drug, but only 0.6 percent, or 43,000, would likely derive a net benefit from it. The rates are lower for black women than white women, Freedman said, because the overall risk for breast cancer in black women is lower and because the rates of stroke, deep vein thrombosis, and pulmonary embolism are higher than among white women. The results for black women, however, are less stable than those for white women because less is known about breast cancer risk and the incidence of some of the side effects in this group, said Freedman.
In examining who would do well on tamoxifen, the researchers found that an overall net positive benefit was related to age. Younger women are less likely than older women to experience the drug’s adverse affects. This means that if a 40-year-old woman and a 60-year-old woman had the same breast cancer risk, the younger woman would likely derive a better overall benefit from the drug.
When the researchers did the age analysis, they found the highest percentage of women likely to benefit overall from tamoxifen were age 40 to 59. More than 8 percent of these women would potentially gain from chemoprevention, compared to 2.1 percent for women age 60 to 69.
In terms of preventing actual breast cancers, the researchers estimated that among the 2.4 million white women who could likely benefit overall from taking tamoxifen, 58,148 breast cancers would develop over the next five years. But, if all these women took the drug for that length of time and experienced the typical 49 percent reduction in breast cancer, 28,492 cases could be prevented or deferred.
Further information on the risks and benefits of taking tamoxifen for breast cancer chemoprevention can be obtained at NCI’s Web site http://cancer.gov/bcrisktool. Women can also call the National Cancer Institute’s Cancer Information Service at 1-800-4-CANCER (1-800-422-6237).
Journal
JNCI Journal of the National Cancer Institute